Anadromous trout threatened by whitefish gill‐net fisheries in the northern Baltic Sea

We estimated the effect of the gill‐net fisheries targeted at whitefish (Coregonus sp.) on anadromous sea trout, Salmo trutta, in the Gulf of Bothnia, Baltic Sea using separate data for fish species. The analysis of sea trout captures was based on tagging and recapture data collected in 1998–2011, while whitefish data were derived from individual samples of commercial fisheries from the same period. The mesh sizes used in gill‐net fishing and the seasonal and temporal distributions of recaptured sea trout and sampled whitefish were compared in the northern and southern Gulf of Bothnia. The trout had typically spent 1–2 years at sea, and they were mainly immature with a median body length of 40–43 cm at the time of recapture in gill nets. Despite the increase in the minimum permitted landing size from 40 to 50 cm in 2008, the median length of recaptured trout remained unchanged during the study period. Most (59%) of the gillnetted trout were caught in the southern Gulf of Bothnia in gill nets with mesh sizes of 40–45 mm, which were also used in the whitefish fishery (72%). In the northern Gulf of Bothnia, nets with a smaller mesh size of 25–39 mm took 83% of the whitefish catch and 39% from recaptured trout. In both areas, the overlap in mesh sizes used to gill‐net catch whitefish and sea trout increased during the study period. There were clear seasonal and areal differences in the relative probability of sea trout being captured in gill nets, suggesting that carefully tailored spatial and temporal restrictions on gill‐net fisheries could provide a tool to protect young sea trout without causing intolerable difficulties for the fisheries targeting other species.     

ASPECTS OF THE FISHERIES OF LAKE LIAMBEZI,CAPRIVI

SUMMARY

Lake Liambezi, situated in the Linyanti Swamp, Caprivi, is inhabited by 43 fish species. Population estimates with seine nets, rotenone and explosives gave values of 74 to 157 kg ha^?1. The composition of seine net catches differed markedly from gill net catches where non-Cichlids form a majority. Results of a fleet of gill nets show that best catches of 25 kg 50 m^?1 net night^?1 can be made with 60 mm mesh nets, catching predominantly non-Cichlids (Marcusenius macrolepidotus, Schilbe mystus, Synodontis spp.). A small commercial fishery is established on the lake, with 60 active fishermen using large mesh (100–150 mm) gill nets. Their catch consists mainly of cichlids (Sarotherodon andersoni and S. macrochir) and Clarias spp. Production figures dropped from 637 ton in 1973–74 to 115 ton in 1975–76. This drop is ascribed to reduced effort as well as a reduction in catch per unit effort. Recommendations are made to utilise the fishery potential of Lake Liambezi in a more balanced way by introducing small mesh gill nets to harvest non-Cichlid populations.     

Effect of artificial baits on the catch efficiency of monofilament gill nets

Greater success in catching fish with gill nets is possible if more the fish are attracted to the nets. Bait has been successfully used to lure fish into the gill nets. With the aim of increasing the catch efficiency of monofilament gill nets, a study was conducted between January 2009 and December 2010 to test the effect of artificial baits on the efficiency of 40, 50 and 55 mm monofilament gill nets in the Keban Dam Lake (Elaz??/Turkey). The catch rate of the baited nets was compared to similar nets without bait. Baited nets caught 482 fish (71.8% of the total, seven species). Non‐baited nets caught 189 fish (28% of the total, seven species). The majority of fish (238) were caught using baited gill nets with 40 mm mesh. The most commonly caught species was Capoeta trutta. The differences between the baited gill nets and non‐baited gill nets were statistically significant (P < 0.05). Finally, it was understood that artificially baited monofilament gill nets had higher catch efficiencies than non‐baited monofilament gill nets. Unlike most natural baits, artificial baits are re‐usable and thus more economical in the long term.     

A comparison of four types of sampling gear used to collect shovelnose sturgeon in the Lower Missouri River

We compared variability in catch per unit effort (CPUE) and size structure of shovelnose sturgeon Scaphirhynchus platorynchus collected in 2003 and 2004 with stationary winter gill nets, drifted trammel nets, hoop nets, and otter trawls in the Lower Missouri River, USA to determine the most precise types of gear to collect all sizes of sturgeon so that refinements of long‐term monitoring protocols can be made. A total of 1947 net sets or trawls collected 8743 shovelnose sturgeon, with 67% of the fish collected during winter gill netting (16% of total samples). Mean coefficient of variation (CV) among all months for juvenile (age 3 and younger; <250 mm fork length) sturgeon was highest for gill nets and lowest for otter trawls (P = 0.0008). Mean CV of subadult and adult shovelnose sturgeon (≥250 mm) was highest in hoop nets compared to other gear types (P = 0.0002). All gear and mesh sizes collected the most common sizes of shovelnose sturgeon (500–600 mm), but only otter trawls and trammel nets collected fish <150 mm. The higher precision of winter gill nets and summer otter trawls led to fewer samples needed to detect changes in CPUE as compared to hoop and trammel nets. Sampling only with types of gear that do not collect younger shovelnose sturgeon may hinder management decisions that rely on recruitment trends to determine the effects of management actions (e.g. channel modifications).     

Longitudinal and Vertical Spatial Gradients in the Distribution of Fish within a Canyon‐shaped Reservoir

The large‐scale spatial distribution of fish was investigated within a morphometrically simple canyon‐shaped reservoir with a single major tributary and a longitudinal trophic gradient (Římov Reservoir, Czech Republic). Samples of fish were taken by Nordic survey gill nets (several mesh sizes from 8 to 70 mm knot to knot) installed as surface nets at several offshore areas located along the longitudinal axis of the reservoir. Surveys were carried out in late summer during 1999–2003. An obvious distribution gradient of fish was revealed along the longitudinal axis of the Římov Reservoir. The total relative fish abundance and biomass (catch per unit effort) decreased considerably from the upstream end of the reservoir toward the dam. Roach (Rutilus rutilus), bleak (Alburnus alburnus) and bream (Abramis brama) comprised the bulk of catches at all areas. Enhanced dominance of bream was observed in the fish assemblage at the uppermost, more eutrophic area of the reservoir. The highest number of fish species and the highest abundance of young‐of‐the‐year fish were also observed in the tributary area. In the downstream part of the reservoir, gill net surveys along the vertical depth profiles indicated that offshore fish occupied mostly the epilimnion. Extreme flood events affected the Římov Reservoir, however, it seemed they had no significant impact on the gradients described. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

插网网目尺寸及采样时间对潮沟游泳动物采样的影响

在长江河口九段沙盐沼湿地的一条潮沟中,我们比较了两种不同网目尺寸（4mm和8mm）的插网（fyke net）及日夜潮对游泳动物群落采样效率的影响。共捕获鱼类16种5476尾、虾2种4982只。采用方差分析、柯尔莫诺夫-斯米尔诺夫非参数检验法和无度量多维标定分析对两种网目插网的捕获效率,样品的体长分布,群落结构及日夜潮间的差异进行了分析。结果表明,网目尺寸对样品中游泳动物的物种数和鱼类的多度没有影响,但是小网目的插网对虾类有较高的捕获率。两种网目尺寸的插网均在夜潮中捕获较多的鱼类个体,但虾类在日潮捕获较多。在日潮中,网目尺寸对捕获物中五种优势游泳动物的体长分布无显著影响。在夜间,小网目的插网对斑尾复缎虎和脊尾白虾的较大个体具有较高的捕获率,但对较大个体鲅的捕获率不如大网目的插网。样品中游泳动物的群落组成在日夜潮间存在明显差异,大网目捕获物的样本间差异比小网目小。     

Different growth trends of whitefish (Coregonus lavaretus) forms in the northern Baltic Sea

The sea growth of two whitefish forms, anadromous (Coregonus lavaretus lavaretus) and sea‐spawning (Coregonus lavaretus widegreni), was analysed using samples collected from the commercial sea catch in the Gulf of Bothnia (GoB) in the northern Baltic Sea during 1998–2014. In the GoB area, these two forms are possible to identify because the gill‐raker number and size at maturity vary between forms. The growth rate of the forms is linked to their feeding area. Sea‐spawning whitefish, which has a feeding migration near its home site, was shorter in the northern GoB (66°N–64°N) at the ages of 3–11 than those in the southern GoB (64°N–60°30′N). In the data, most whitefish were caught with gill nets in the GoB. The mesh sizes of gill nets capturing the anadromous form were mostly 35–45 mm, while those capturing the sea‐spawning form were <35 mm in the northern GoB. It is likely that the different growth trends for small and large whitefish were connected with differences in their recruitment for fishing. The length of anadromous females at the age of four sea years increased significantly, but the length of six‐year‐old anadromous female whitefish decreased over the catch years from 1998–2014. In contrast, the length of slow‐growing sea‐spawning whitefish of six years or older increased significantly in relation to the catch year in the gill‐net catch. The increase in the growth of young age groups in both forms was probably associated with the increasing temperature and the low fishing pressure on small fish. The decreasing age at capture for both forms and the depression of the mean size of old anadromous whitefish are signs of high fishing pressure with a high gill‐net effort that selectively removes the largest and oldest individuals of both forms.     

Length–weight relationships for 13 fish species in the Tangwang River,a tributary of Songhua River,China

The present work provides length–weight relationships (LWRs) for 13 fish species belonging to six families. The fishes were collected with gill nets (mesh sizes 10, 20, 40, 60, 90, 120 mm) and set nets (5, 8, 10 mm mesh sizes) from April 2012 to September 2015.     

Length-weight relationships of six fish species of tropical lake,Mexico

The length–weight relationships (LWRs) for six native fish species from the Chapala Lake, Mexico, are described in this paper. The fish were caught monthly with the support of local fishermen from different landing sites. The daily catch is obtained using the nasa charalera (pot) and the gill net. Each fisherman can operate 20 pots daily at each fishing site for approximately six or seven months; this pot is a 2 m long cylinder, 0.5 m in diameter, with a mesh opening of 0.5 to 1 cm. Also, each fisherman can set between 10 and 20 gill nets each day at sunset and recover them in the morning, again for six or seven months. The length of the gill net is 30 m or more with a 2 m drop, with a 1 cm mesh size. The study provides basic data, morphometric data on length and weight of native species belonging to the genus Chirostoma. These species are of great importance for the local fishery with the Mesa Central of Mexico.     

Gillnet selectivity and the efficiency of alternative combinations of mesh sizes for some freshwater fish

The selectivity curves of nylon gillnets for burbot and two groups of Arctic charr of different fatness were found by the Gulland-Harding method. They are compared to the selectivity curves of whitefish, brown trout, and perch. Based upon these curves and the relationships between most efficient mesh size and fish length, the pooled relative efficiency of a geometrical and an arithmetical series of mesh sizes was calculated for salmonids, burbot, and perch. This efficiency of the geometrical series is different for the three groups of fish, but equal for each group over the interval between the modal lengths of the smallest and largest mesh sizes. The efficiency of the arithmetical series is also different for the three groups, increases linearly with fish length, and is valid over the interval between the modal lengths of the second smallest and the second largest mesh sizes. Each series has its special advantages, and both are easily adjusted to the fish length interval of interest. They are especially adapted for the catch of salmonids. For these, the variations of condition factor is more important than any characteristics of the species, and its influence upon the most efficient mesh size to fish length relationship and the pooled relative efficiency of the two series of nets is evaluated.     

An initial assessment of sampling procedures for juvenile pallid sturgeon in the Missouri River downstream of Fort Randall Dam, South Dakota and Nebraska

We compared the effectiveness of passive gill nets, hoop nets, set lines, and drifted trammel nets, towed beam trawls and otter trawls to develop criteria to best determine the mean catch per unit effort (CPUE) for juvenile pallid sturgeon (Scaphirhynchus albus) based on selectivity and seasonal efficiency in various habitats of the Missouri River downstream of Fort Randall Dam, South Dakota. Sampling occurred from April to November in 2003 and 2004 and from March to November in 2005. We captured 29 juvenile pallid sturgeon in a total of 498 overnight gill net sets, 55 in 870 drifted trammel nets, 19 on 1683 set lines, and six in 166 otter trawl tows. No pallid sturgeon were captured in 515 beam trawl tows or 520 overnight hoop net sets. Seasonal trends in mean CPUE were found and the relative precision was the greatest in October and November for gill nets, in August for trammel nets, in April for set lines, and in October for otter trawls. A higher proportion of pallid sturgeon captures for gill nets were in the inside bend macrohabitat generally associated with lower water velocities, trammel nets over sand substrate and in the outside bend macrohabitat typically associated with higher water velocities and greater depths, set lines in lower water velocities, and otter trawls in depths >2.5 m and over sand substrate. Although we found trends among seasons and habitats for gill nets, trammel nets, set lines, and otter trawls, the catch rates were low and annual point estimates of relative abundance are not adequate to detect changes in relative abundance of juvenile pallid sturgeon in this reach of the Missouri River. Independently, gill nets, trammel nets, and otter trawls likely captured the size structure of the population of pallid sturgeon in the Missouri River downstream of Fort Randall Dam. Based on our results, a standardized protocol can now be established to systematically monitor juvenile pallid sturgeon, an essential element for determining responses to recovery efforts in the Missouri River.     

Direct estimates of the selectivity of a multimesh and a series of single gillnets for brown trout

Hatchery brown trout Salmo trutta (1406) of length 5–20 cm were released into a 7.5 ha shallow lake. They were recaptured with 28 multimesh gillnets of a type proposed for use in the Nordic countries and 42 single gillnets of mesh size 8–22 mm (knot to knot). Corrected for the effect of different twine thicknesses, the single nets showed the highest selectivity for 16.5 mm and smaller meshes and the multimesh nets for the larger meshes. The height of the selectivity curves increased exponentially with mesh size, and most distinctly for the multimesh nets. Over a limited range of mesh sizes the selectivity of the two types of net did not differ much. Extended over mesh sizes 10.0–55.0 mm, the catching abilities of the two types of net became considerably different.     

浅水湖泊四种渔具捕捞选择性及对鱼类群落影响比较研究

了解不同种类和大小的鱼类对各种捕捞方式的脆弱性, 对于制定最佳的捕捞策略和合理开发鱼类资源至关重要。研究从渔获物组成、捕捞效率和个体大小等方面, 对长江中游典型湖泊牛山湖的四种渔具(网簖、刺网、电捕和鸬鹚)的捕捞选择性进行了比较分析。使用相对重要性指数(IRI)评价渔具捕捞对鱼类物种的选择性, 同时通过计算渔获物的平均营养水平(MTL)和饵料鱼与凶猛性鱼类的重量比, 分析捕捞对鱼类群落营养结构的潜在影响。结果表明, 网簖和电捕捕获的鱼类种数多、体长范围广, 对鱼类资源的破坏性最大, 应当加以限制或废除; 鸬鹚是捕获鳜(尤其是高龄鳜)的一种有效方式, 但对幼鳜和其他幼鱼也有较大损害, 应控制其捕捞强度; 刺网主要捕获几种经济鱼类, 并可平衡捕食者和饵料鱼之间数量关系, 是利用鱼类资源的一种较好方式, 关键在于对刺网网目大小和捕捞强度的限制。建议湖泊捕捞应从单一物种转向多物种综合管理, 并考虑物种之间的营养生态关系, 以更好地保护和可持续利用渔业资源。     

Gear type,species composition and economic value of fisheries in the mangroves of Pak Phanang,Thailand

Fishing gear and methods, catch composition, annual catch numbers and the monetary value of fisheries based in the mangroves of Pak Phanang, Nakhon Si Thamarat province, Thailand, were studied from September 2004 to February 2005. The fishing gear and methods were surveyed by visual observation and interviews. Sample catches were taken for each type of fishing gear and method. Semi-closed questionnaire surveys were conducted among fishers, local traders, local administrative officers and related organizations to derive information on utilized species, gear and methods, fisher and fishery status, and trading and catch price. The Pak Phanang mangrove fishery has three types of multi-species capture gear: channel traps, gill nets and lift nets. It also has three kinds of single-species (group) gear/methods: crab traps, catfish hooks and hand capture using a long tail fishing boat. A total of 57 fish species in 27 families, and 23 shell fish species in 8 families were recorded from the catch samples. Penaeid shrimp (25%), and ambassid (31%) and mugilid (24%) fishes were the abundant groups in the channel traps, while ariid (36%) and mugilid (19%) fishes were abundant in the lift net catches. Ambassid (42%) fish dominated the gill net catches. Species richness (number of species) varied depending on sampling month and type of fishing gear. The total annual catch and its monetary value were estimated to be 442–551 tons and US $368,038–733,973, respectively. The estimated annual catch per unit area of mangrove was 63–79 kg ha⁻¹, which generated a market value of US $368,038–733,973, respectively. The estimated annual catch per unit area of mangrove was 63–79 kg ha⁻¹, which generated a market value of US 52–105.     

Factors influencing dolphin depredation in coastal fisheries of the northern Aegean Sea: Implications on defining mitigation measures

Dolphin interactions with coastal fisheries are of major concern, reportedly leading to gear damage, which increases the cost of coastal fishing globally and in the Mediterranean Sea. The aim of this study was to determine the effect that gear, target species, mesh size, depth, soaking duration, fishing area, and season have on net depredation frequency and to offer insights on possible mitigation solutions. From November 2013 to February 2016 we monitored 107 active coastal fishers in 22 ports of the northern Aegean Sea coastline, identified the main target species of the fishery and recorded the damages on gill nets and trammel nets caused by dolphins, mainly the common bottlenose dolphin (Tursiops truncatus). Quasi-binomial generalized linear models were used to determine the relationship between the examined factors and depredation frequency. The analysis revealed that the gears mostly depredated were gill nets and trammel nets with small mesh sizes, mainly targeting surmullet (Mullus surmuletus), red mullet (Mullus barbatus), common sole (Solea solea), European hake (Merluccius merluccius), and caramote prawn (Melicertus kerathurus). The probability of depredation was also significantly dependent on the fishing area.     

The variability of the catch from gill nets set for pike Esox lucius L.

The variability of the catch of pike Esox lucius L. from gill nets in Windermere over the period 1961–71 has been analysed. Altogether 2952 catches have been treated by analysis of variance after an appropriate transformation. A representative value of the residual between catch variance has been used to estimate the reliability of the means of catches based on different numbers of net settings. It is shown that many settings of gill nets are required to get a reliable mean catch.     

Biomass and Abundance Biases in European Standard Gillnet Sampling

The European Standard EN 14757 recommends gillnet mesh sizes that range from 5 to 55mm (knot-to-knot) for the standard monitoring of fish assemblages and suggests adding gillnets with larger mesh sizes if necessary. Our research showed that the recommended range of mesh sizes did not provide a representative picture of fish sizes for larger species that commonly occur in continental Europe. We developed a novel, large mesh gillnet which consists of mesh sizes 70, 90, 110 and 135mm (knot to knot, 10m panels) and assessed its added value for monitoring purposes. From selectivity curves obtained by sampling with single mesh size gillnets (11 mesh sizes 6 – 55mm) and large mesh gillnets, we identified the threshold length of bream (Abramis brama) above which this widespread large species was underestimated by European standard gillnet catches. We tested the European Standard gillnet by comparing its size composition with that obtained during concurrent pelagic trawling and purse seining in a cyprinid-dominated reservoir and found that the European Standard underestimated fish larger than 292mm by 26 times. The inclusion of large mesh gillnets in the sampling design removed this underestimation. We analysed the length-age relationship of bream in the Římov Reservoir, and concluded that catches of bream larger than 292mm and older than five years were seriously underrepresented in European Standard gillnet catches. The Římov Reservoir is a typical cyprinid-dominated water body where the biomass of bream > 292mm formed 70% of the pelagic trawl and purse seine catch. The species-specific relationships between the large mesh gillnet catch and European Standard catch suggested that the presence of carp (Cyprinus carpio), European catfish (Silurus glanis), tench (Tinca tinca) or bream warrants the use of both gillnet types. We suggest extending the gillnet series in the European Standard to avoid misinterpretation of fish community biomass estimates.     

Evaluating catches of salmonids taken by gillnets

Data from 10 185 gillnetted brown trout ( Salmo trutta L.) and 5777 Arctic charr [ Salvelinus alpinus (L.)] were analysed. Relative to mesh size, the most efficiently caught fish length (the modal length) was somewhat larger with floated nets than with smaller nets placed on the bottom. There were no such consistent differences between the two species. Fish that exceeded the modal lengths were caught more efficiently than smaller fish. The relationship of most efficient mesh size to fish length and condition factor fitted excellently to a linear equation. Based on this equation, the selectivity curve for Arctic charr gillnetted in the Nesjø reservoir, the different efficiency of nets of 0.1 and 0.17mm twine and an equation expressing the increasing height of the selectivity curve with increasing mesh size, a model is presented to compare length frequencies and total numbers of gillnet catches taken with any combination of mesh sizes.     

Catch composition of drift gillnets in a multi-species fishery with an emphasis on kingfish (Scomberomorus commerson) in Northern Persian Gulf

Gillnets are the primary fishing gear used in tropical multi-species fisheries along the Iranian southern coastal waters. Therefore, it is necessary to investigate the catch composition and performance of fishing gear for the possible negative effects on the stocks. Catch data and length frequency distribution (just for Kingfish, S. commerson) were examined for three kinds of drift gillnets: 1) small-meshed nets, 2) medium-meshed nets, and 3) nets with a single mesh size of 165 mm. This study was conducted in the northern part of the Persian Gulf from 2014 to 2015. Overall, 60 species of 32 families from four fish groups (i.e., Pelagic, Demersal, Benthopelagic, and Reef-associated) were recorded. Each net-type was used to target more than one species, with most species being considered as bycatch. Commercial fish species were caught the most, accounting for around 83% of the total catches. S. commerson is targeted by all kinds of nets, with medium-meshed nets being most efficient. The majority of S. commerson caught by the nets were immature. Overall, the total ban of gillnets with small mesh sizes is recommended to protect young S. commerson and prevent the possible risk of growth overfishing. Studies on gillnet selectivity are needed to set minimum mesh regulations for S. commerson and other commercial species if the potential impacts of gillnet are known.     

Catch composition and catch per unit effort of filter net fishery in Jordan,Guimaras, central Philippines: Its implication for management

In Jordan, Guimaras, central Philippines, the use of filter net is prevalent. This fishing gear uses fine‐mesh netting to capture small organisms such as anchovies and small shrimps of the genus Acetes. However, the use of fine‐mesh nets sparked issues on conservation and overfishing. Therefore, this study was conducted to determine the catch per unit effort and catch composition of filter nets in Jordan, Guimaras during the summer season of 2016. The total length of the gear was 23–40 m with a mouth opening of 11 m × 11 m. The cod end had the smallest mesh size (3 mm) and was provided with a non‐return valve. It was set by 1–3 persons during periods of strong current. Results showed that the amount of catch increased with the increase in current speed with a correlation value of 0.62. CPUE for the sampling period ranged from 21 to 25 kg per operation. Furthermore, the amount of catch was more than 90% juvenile and fish larvae. Based on the responses of filter net owners, the months of February to May were considered lean season due to insufficient amount of catch while the peak season occurs during the months of September to January when the catch of anchovy and Acetes were abundant. Clearly, the use of this gear showed some ecological implications due to the catch of juvenile and fish larvae. However, fishers benefit from adult fish as well as both juvenile organisms and fish larvae and both national and local polices currently do not define the allowable percentage of these organisms from the total catch in order to be considered ecologically sensible. Aside from providing baseline data, these results are essential in amending existing policies or formulating new ordinances that could help in regulating the use of filter net in Jordan, Guimaras.